This invention relates to an improvement in low noise electrolytic capacitors suitable for use in the signal transmission circuit and signal circuit of an audio apparatus, for example.
As is well known in the art, the prior art electrolytic capacitor for use in audio apparatus or the like comprises an anode foil with one surface etched to become coarse and is provided with an oxide film, a spacer comprising low density capacitor paper and a cathode foil which are laminated and then wound into a roll. The capacitor paper acts as an insulator to insulate the anode foil and the cathode foil from each other and is impregnated with an electrolyte.
When such electrolytic capacitor is used in an audio device, for example, the oxide film of the anode foil acts as a dielectric to provide a large electrostatic capacitance, whereas the electrolyte cooperates with the cathode foil to act as a type of an electrode conductor. Accordingly, the electrolyte and the spacer which are interposed between the anode foil and the cathode foil form a portion of the current path for an audio signal or the like to impart various influences upon the signal passing therethrough. More particularly, the resistance component of the electrolyte is directly related to the impedance characteristic and the tan .delta. characteristic of the capacitor. Theoretically, it is desirable to use an electrolyte having a high electroconductivity but such electrolyte has a tendency of corroding the electrode film made of aluminum for example. For this reason, an electrolyte consisting of a solution of ammonium borate having a conductivity of about several hundred ohms/cm is used practically. The electric charge migrates through such electrolyte so that, when an alternating current signal is passed, the electrolyte presents the following problem.
More particularly, the spacer containing the electrolyte and interposed between the oxide film formed on the surface of the anode electrode and the cathode electrode manifests said resistance component and the resistance of the spacer varies nonlinearly depending upon the distance between the electrode surfaces and the applied voltage. Thus, the potential distribution does not always vary linearly as in a conventional resistor. Accordingly, the spacer has a nonlinear influence upon the movement current inside the capacitor with the result that many higher harmonic components are induced in the signal. Of course, the nonlinear characteristic formed between the oxide film and the electrolyte can not be ignored, and the nonlinearlity is enhanced by the aforementioned characteristic of the electrolyte.